Allocation of the calls entered by elevator users to different elevators in an elevator system is one of the basic functions of the control of the system. The aim of allocation is to assign the calls to the elevator cars so as to optimize a desired parameter descriptive of the operating capacity of the elevator system. Traditionally, the most commonly used parameters include e.g. passenger waiting times and traveling times. Typically, from these times, mean values are calculated and distributions are determined. ‘Calls’ refers generally to all calls issued, i.e. both calls entered using up/down buttons on different floors and destination floor calls given in the elevator car. The former are landing calls and the latter are car calls. In addition, calls may be given using call input devices consistent with the so-called destination control method. In the destination control method, the elevator user lets the system know his/her traveling destination floor already in the elevator lobby via a call device, and in this case no separate call has to be input in the elevator car.
There are various call allocation methods, and each elevator manufacturer has its own methods for implementing cost-effective call allocation satisfying the elevator user. Each method naturally involves a number of characteristic parameters, which are used to influence the operation of the method. The control can be arranged e.g. so that in different traffic situations the parameter set best suited to each situation is employed. The aim of this is to allow the elevator system to adapt its operation as appropriate with respect to the prevailing traffic situation. An exceptional traffic situation may be e.g. a peak traffic condition, during which the system registers many simultaneous landing calls.
An efficient prior-art elevator allocation method is the use of genetic algorithms, especially in systems comprising several elevators. Genetic algorithms are described e.g. in Finnish patent specification FI112856B. Genetic algorithms do not guarantee that the absolutely optimal value is found, but the results obtained in practical applications are very close to that. In genetic algorithms, elevator routes can be encoded into different chromosomes, in which one gene determines an elevator customer and the elevator serving him/her. The position of the gene in the chromosome specifies the call, and the gene value tells the elevator serving the call. The system sets out e.g. from a randomly selected route alternative, to which are applied various genetic procedures, such as proliferation, crossbreeding and mutation. One generation at a time, new chromosomes are produced by these genetic procedures, and at the same time the chromosomes thus obtained are analyzed to decide whether they are eligible for further processing. Eligibility may mean, for instance, that a waiting time below a given value is obtained. Crossbreeding means combining two route alternatives at random to create one new route alternative. In mutation, the values of the genes of the chromosome are varied at random. At some stage, the chromosome results given by the algorithm converge, and from the last set of chromosomes processed, the best one in respect of eligibility is selected. The passengers are allocated to the elevators in accordance with the genes of the best chromosome.
The elevator system has to include precautions in case of unexpected interruption of the supply of electricity. When the normal power supply fails, the stand-by power generator of the building starts running—if the building has one. The stand-by power is normally not sufficient for the needs of the entire elevator group, but traditionally emergency power drive (EPD) of elevators is implemented by beforehand selecting the elevator or elevators to serve passengers during emergency operation.
When the power supply fails, the elevator with the passengers may stop between floors. After the emergency power generator has started running, the elevator group control system returns the elevators one by one in a previously defined order to a return floor (generally a lobby), where the passengers can get out of the elevator. After this returning operation, the above-mentioned predetermined elevators are put into normal service (so-called “full service lifts”). The number of such elevators to be taken in use depends on the power capacity of the emergency power generator and on the amount of power required by the elevators at the worst. The loads of the elevator car and counterweight is almost always unbalanced, and moving the elevator in the so-called light direction (empty car upwards, full car downwards) requires less power than moving it in the so-called heavy direction (empty car downwards, full car upwards). Present elevator drives are even able to restore potential energy stored in passengers back into the electric network, i.e. to function as generators when driving in the light direction or when the elevators are being decelerated.
FIG. 1 presents as an example of prior art a group of three elevators 10, 11, 12, where elevator ‘L1’ 10 is an elevator serving passengers in a situation of EPD operation. In this example, the speed of the elevator is 2.5 m/s, acceleration 0.8 m/s2 and floor height 3.2 m. To reduce passenger waiting times, riding times of different types associated with elevator operation can be determined. These are presented in Table 1.
TABLE 1Elevator riding timesStage of operationtime [s]Short ride (acceleration + deceleration)4Acceleration to full speed3Deceleration from full speed3Passage through floor at full speed1.25Stop at floor10
In the situation presented in FIG. 1, two calls are active, up calls at floors five and six (calls “U5” 13 and “U6” 14), from both of which one passenger is going to floor nine. On the basis of Table 1, the waiting time obtained for the active calls U5+U6 is 6.5 s+20 s=26.5 s.
Table 2 lists examples of power consumption during different stages of elevator operation with three different loads. The power consumption values are based on real data measured in connection with the use of a V3F-80 as power source.
Table 2. Power requirements of upward and downward travel with different loads. Pacc is power consumption during acceleration, Pspd is power consumption during constant-speed operation and Pdec is power consumption during deceleration.
Assumed passenger mass 75 kgNumber ofpassengersLoad [kg]Pacc [kW]Pspd [kW]Pdec [kW]Upward travel005.62.50.41756.42.50.421507.32.40.432258.42.30.4Downward travel0021.916.73.317519.913.93.3215018.011.53.3322516.49.53.3
FIG. 2 presents the power requirement of the elevators of FIG. 1 in one route alternative as a function of time. Since only elevator L1 is running, the total power consumption (Sum) of the elevator group is the same as the power consumption of elevator L1.
In the example, the maximum power required is 21.9 kW as an empty elevator is accelerating downwards (in the heavy direction), but this power value is smaller than the maximum power capacity of the emergency power generator.
Power and energy consumption are two different facets of resource management, where power is an instantaneous quantity whereas energy is a cumulative quantity. There are prior-art solutions where energy consumption is included in route optimization. Patent specification WO 02/066356 describes a system for controlling an elevator system wherein the energy consumed by the elevator system is minimized in such a way that a desired requirement regarding elevator passengers' service time is fulfilled on an average. In this method, a given service time of the elevator group is given a target value for call allocation. The service time used may be e.g. call time, passenger waiting time, traveling time or riding time.
Prior art is also represented by specification FI115130, which is an extension to the method description in specification WO 02/066356.
In other words, the control method optimizes two non-commensurable quantities of different types, i.e. waiting time and energy consumption. To make these quantities commensurable and mutually comparable, elevator routes R are selected in the method according to specification WO 02/066356 so as to minimize the cost termC=WTTN(R)+WEEN(R)  (1)
TN(R) is a normalized sum of call times for route alternative R, and correspondingly EN(R) is the normalized energy consumption caused by route alternative R. WT and WE are the weighting coefficients of the aforesaid cost terms, so that0≦WT≦1 and WE=1−WT.  (2)
Prior-art methods are designed to find routes on which the passenger waiting time produced by the elevator group and the power consumption of the elevators are suitably balanced. However, optimization of energy consumption does not guarantee that the elevators thus routed will not at some stage e.g. accelerate simultaneously in the so-called heavy direction. In other words, along the route there may occur large power spikes even if the total energy consumption for the route alternative in question is below the defined upper limit.
As another example of prior art, and referring to the situation illustrated in FIG. 1, the best alternative in respect of passenger waiting times would be for elevator 1 to pick the call from floor 6 and for elevator 3 to serve the call from floor 5. This elevator routing alternative is presented in FIG. 3. The system comprises three elevators, elevator L1 30, elevator L2 31 and elevator L3 32. Calls currently active are an up call (U5) 33 from the fifth floor and an up call (U6) 34 from the sixth floor. The elevator movements are as shown in FIG. 3. It is to be noted that in this example both passengers having entered a call want to get to floor nine. In this situation, elevator L2 31 remains at rest and is not involved in serving the calls. The power requirement according to this routing is illustrated in FIG. 4. As can be seen from FIG. 4, in the best route alternative in respect of waiting times, the power required in the early part of the routing exceeds the capacity of the emergency power generator. The overall waiting time in this routing alternative is 4.5 s+7.5 s=12 s. Unfortunately, this routing alternative is not acceptable because the maximum power of the emergency power generator is exceeded.